1. Technical Field of the Invention
The present invention relates generally to an improvement on a gas sensor which may be employed in an oxygen measuring device of an air-fuel ratio control system measuring an oxygen content in exhaust gasses of an internal combustion engine of automotive vehicles.
2. Background Art
For burning control of fuel in internal combustion engines, modern automotive vehicles use a gas sensor, e.g., an oxygen sensor which is installed in an exhaust system to measure the concentration of oxygen in exhaust gasses. A typical gas sensor of such a type includes a first insulation porcelain which is installed in a cylindrical housing and has disposed therein a sensing element and a second insulation porcelain abutting on the first insulation porcelain in alignment with the longitudinal center line of the housing. The first insulation porcelain rests on an annular step formed on an inner wall of the cylindrical housing through a packing ring. The second insulation porcelain is arranged in contact of an end with an end of the first insulation porcelain.
Usually, insulation porcelains are made of an insulating ceramic material. The ceramic is resistive to compression, but weak in tension. The ceramic is also subject to thermal expansion and shrinkage during baking, which may result in formation of waves on the surface thereof. This will cause a contact area between the ends of the first and second porcelains to be shifted laterally from the packing ring, thereby producing a bending stress (i.e., tensile stress) acting on the first insulation porcelain, which, in the worst case, leads to breakage of the first insulation porcelain.
For avoiding the above problem, Japanese Patent First Publication No. 11-242013 teaches a cushion which is disposed between the first and second insulation porcelains, however, this will result in increases in production process and cost.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide a gas sensor designed to minimize the breakage of an insulation porcelain during assembly and use of the gas sensor.
According to one aspect of the invention, there is provided a gas sensor which comprises: (a) a hollow cylindrical housing having a given length, the housing having a seat surface formed on an inner wall thereof: (b) a first insulation porcelain having a gas sensor element installed therein, the first insulation porcelain being disposed on the seat surface of the housing through a packing ring; and (c) a second insulation porcelain disposed in contact of an end surface thereof with an end surface of the first insulation porcelain, a contact between the first and second insulation porcelains being located within a contact permissible range defined by projecting outer and inner edges of the packing ring onto the end surface of the first insulation porcelain.
In the preferred mode of the invention, one of the first and second insulation porcelains has a protrusion formed on the end surface thereof to establish the contact between the first and second insulation porcelains.
One of the first and second insulation porcelains may have at least one ridge formed on the end surface thereof to establish line contact between the first and second insulation porcelains.
The contact permissible range may be defined by a width of the packing ring within an area ranging over an interval of the width of the packing ring from a center line middle between outermost and innermost edges of the packing ring in an inward direction of the gas sensor and an interval of the width of the packing ring from the center line in an outward direction of the gas sensor.
According to the second aspect of the invention, there is provided a gas sensor which comprises: (a) a hollow cylindrical housing having a given length, the housing having a seat surface formed on an inner wall thereof: (b) a first insulation porcelain having a gas sensor element installed therein, the first insulation porcelain being disposed on the seat surface of the housing through a packing ring; and (c) a second insulation porcelain disposed at an end surface thereof on an end surface of the first insulation porcelain at two contacts, a middle between the contacts being located within a contact permissible range defined by projecting outer and inner edges of the packing ring onto the end surface of the first insulation porcelains.
In the preferred mode of the invention, the contact permissible range may be defined by a width of the packing ring within an area ranging over an interval of the width of the packing ring from a center line middle between outermost and innermost edges of the packing ring in an inward direction of the gas sensor and an interval of the width of the packing ring from the center line in an outward direction of the gas sensor.